The high-school I went to served pizza and french fries regularly. The food was extremely unhealthy and to my knowledge that hasn't changed much. It's far easier to deep fry a few hundred pounds of tater tots than to make a few thousand sandwiches and sliced apples.

I doubt either option was fairly healthy for the students, but I wouldn't use a high-school cafeteria as the epitome of healthy living.

Also for some reason the rich kids at my school typically brought their own lunch and ate more natural food than the middle-class or the poor kids. Anecdotal at best, but that was my experience.